This invention relates to a fluidized bed reactor and method for operating same and, more particularly, to a fluidized bed reactor including a stripper-cooler located adjacent the reactor for controlling the circulation and the temperature of the bed material.
Reactors, such as combustors, steam generators and the like, utilizing fluidized beds as the primary source of heat generation, are well known. In these arrangements, air is passed into the furnace section of the reactor and through a bed of particulate material contained therein which includes a mixture of a fossil fuel, such as coal, and an adsorbent, such as limestone, to adsorb the sulfur generated as a result of combustion of the coal. The air fluidizes the bed and promotes the combustion of the fuel. When the reactor is utilized as a steam generator, the heat produced by the combustion of the fuel is utilized to convert water to steam. Fluidized bed reactors provide an attractive combination of high heat release, high sulfur adsorption, low nitrogen oxides emissions and fuel flexibility.
The most typical fluidized bed combustion system is commonly referred to as a "bubbling" fluidized bed in which a dense bed of the particulate material is supported by an air distribution plate, to which the combustion supporting air is introduced through a plurality of perforations in the plate, causing the particulate material to expand and take on a suspended, or fluidized, state. The air velocity is typically two to three times that needed to develop a pressure drop which will support the bed weight (e.g., minimum fluidization velocity), causing the formation of bubbles that rise up through the bed and give the appearance of a boiling liquid.
In an effort to extend the improvements in combustion efficiency, pollutant emissions control, and operation turn-down afforded by the bubbling bed, a fluidized bed reactor was developed utilizing an expanded and elutriating fluidized bed commonly referred to as a "circulating" fluidized bed. In these arrangements, the size of the particulate material is decreased and/or the mean air velocity is increased when compared to the bubbling bed, so that the bed surface becomes more diffused and the entrainment of solids from the bed is increased. According to this process, in the lower portion of the furnace section, fluidized bed densities are attained which are well below those typical of bubbling fluidized beds, whereas the upper portion of the furnace section becomes loaded with entrained particulate material, or solids, to a much greater extent than in bubbling fluidized beds. This increased solids entrainment in the upper portion of the furnace section results in a high solids throughput which requires a high solids recycle rate. Reactors having high solids recycle rates require large and expensive separators to separate the entrained solids from the hot combustion gases before the gases pass through a heat recovery area to reduce erosion of the heat recovery surfaces in the heat recovery area. The separated solids are passed back to the fluidized bed.
U.S. Pat. Nos. 4,809,623 and 4,809,625, assigned to the same assignee as the present application, disclose a fluidized bed reactor in which a dense, or bubbling, fluidized bed is maintained in the lower portion of the furnace section, while the bed is otherwise operated as a circulating fluidized bed. The design is such that advantages of both a bubbling bed and a circulating bed are obtained, not the least significant advantage being the ability to utilize particulate fuel material extending over a greater range of particle sizes.
In all of these designs, a homogenous mixture of fuel and adsorbent particulate material is formed with a portion of the fuel particles being unburned, a portion being partially burned, and a portion being completely burned. Similarly a portion of the adsorbent is unreacted, a portion is partially reacted and a portion is completely reacted. The particulate material must be discharged from the system efficiently to accommodate the introduction of fresh fuel and adsorbent. To this end, a portion of the particulate material is usually passed from the lower portion of the bed through a drain pipe to remove that portion from the reactor.
It has been found, however, that the particle size distribution in a fluidized bed, an important operating parameter, can be effectively controlled by recirculating part of this removed particulate material back to the furnace section. This is often accomplished by blowing air through the removed particulate material to strip away and entrain the finer portions of the particulate material and returning them to the furnace section.
For example, in U.S. Pat. No. 4,829,912, assigned to the same assignee as the present application, a method of controlling the particle size distribution in a fluidized bed reactor is disclosed in which air entrains the finer portions of the particulate material removed through the drain pipe by stripping them with a stream of air and recirculating them back to the furnace section. In these types of arrangements, the heat of the nonrecirculated particulate material can be put to productive use, such as to preheat combustion supporting gas or for reheat or superheat duty.
A stripper-cooler located adjacent the furnace section of the reactor can accomplish both the recirculation of the finer portions of the removed particulate material and the removal of heat from the removed but nonrecirculated particulate material. In these types of arrangements, the stripper-cooler receives the particulate material from the furnace section and air is blown through a first section of the stripper-cooler to strip, or entrain, some of the finer portions of the particulate material which are returned to the furnace section. The remaining particulate material in the stripper-cooler is then usually passed to a cooler section where heat is removed from the particulate material by passing water/steam in a heat exchange relation to the particulate material or by blowing air through it before it is discharged from the system. When air is used to remove the heat from the nonrecirculated particulate material, this air is often returned to the furnace section as preheated, combustion-supporting air.
However, in some situations, such as when fuels that generate an excessive amount of relatively fine ash are used, or when a relatively large amount of relatively fine adsorbent has to be used with fuels having a relatively high sulfur content, the relatively fine particle material stripped in the stripper-cooler and returned to the furnace section increases the volume of the fines in the furnace section, or the upper furnace loading, to unacceptably high levels. Excessive upper furnace loading requires larger and more expensive stripper-coolers and separators and/or requires that the furnace be operated at a low stoichiometric condition, which is inefficient.
This upper furnace loading is made worse when the method used to cool the particulate material in the cooler section of the stripper-cooler is by blowing air through it. To achieve a high cooling rate and to prevent agglomeration of the particulate material in the stripper-cooler, the air velocity and flow rate through the cooler section must be relatively high. A high air velocity and flow rate, however, entrains greater amounts of particulate material resulting in an even greater volume of fines returned to the furnace section when this air is used as combustion supporting air, thereby further increasing the upper furnace loading.
Also, these types of stripper-coolers normally operate in a continuous mode, i.e., bed material from the reactor is continuously fed to the stripper-cooler. This results in a relatively short residence time of the material in the stripper-cooler and causes residual combustibles to accumulate in the bed ash which is ultimately drained from the system.
Another problem arises in these type stripper-coolers, especially ones that are partitioned, in connection with the processing of relatively large fuel materials, such as waste materials, tramp materials, etc. More particularly, the large materials tend to accumulate in the lower portion of the fluidized bed in the furnace section and thus, for the most part, are not circulated through the stripper-cooler. Also, those relatively large materials that are passed to the stripper-cooler often become jammed or clogged by the partition walls of the stripper-cooler.